Galaxies in Collision

March 1,
2000 -- Pictures of a colossal cosmic "weather system"
produced by the collision of two giant clusters of galaxies have
been captured by NASA's Chandra X-ray Observatory. For the first
time, the pressure fronts in the system can be traced in detail,
and they show a bright, but relatively cool 50 million degree
central region embedded in large elongated cloud of 70 million
degree gas, all of which is roiling in a faint "atmosphere"
of 100 million degree gas.

Right: The two large elliptical galaxies
indicated by arrows in the center of this optical image are thought
to be the central galaxies of two merging galaxy clusters. Together,
the two merging clusters form a single object astronomers call
Abell 2142. The optical image is overlaid with X-ray brightness
contours. The bright source in the upper left is an active galaxy
in the cluster.

"We can compare this to an intergalactic cold front,"
said Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics,
Cambridge, Mass. and leader of the international team involved
in the analysis of the observations. "A major difference
is that in this case, cold means 70 million degrees."

The gas clouds are in the core of a galaxy cluster known
as Abell 2142. The cluster is six million light years across
and contains hundreds of galaxies and enough gas to make a thousand
more. It is one of the most massive objects in the universe.
Galaxy clusters grow to vast sizes as smaller clusters are pulled
inward under the influence of gravity. They collide and merge
over the course of billions of years, releasing tremendous amounts
of energy that heats the cluster gas to 100 million degrees.

The Chandra data provides the first detailed look at the late
stages of this merger process. Previously, scientists had used
the German-US Roentgen satellite to produce a broad-brush picture
of the cluster. The elongated shape of the bright cloud suggested
that two clouds were in the process of coalescing into one, but
the details remained unclear. Chandra is able to measure variations
of temperature, density, and pressure with unprecedented resolution.

Left: Chandra X-ray Observatory
image of the galaxy cluster Abell 2142. For the first time, the
pressure fronts in a system of colliding galaxy clusters can
be traced in detail, and they show a bright, but relatively cool
50 million degree central region (white) embedded in large elongated
cloud of 70 million degree gas (magenta), all of which is roiling
in a faint atmosphere of 100 million degree gas (faint magenta
and dark blue). The scale bar in the lower right hand corner
indicates one minute of arc, which corresponds to approximately
350,000 light years at the distance of Abell 2142.

"Now we can begin to understand the physics of these mergers,
which are among the most energetic events in the universe,"
said Markevitch. "The pressure and density maps of the cluster
show a sharp boundary that can only exist in the moving environment
of a merger."

With this information scientists can make a comparison with computer
simulations of cosmic mergers. This comparison, which is in the
early stages, shows that this merger has progressed to an advanced
stage. Strong shock waves predicted by the theory for the initial
collision of clusters are not observed. It appears likely that
these sub-clusters have collided two or three times in a billion
years or more, and have nearly completed their merger.

These observations were made on August 20, 1999 using the
Advanced CCD Imaging Spectrometer (ACIS). The team involved scientists
from Harvard-Smithsonian; the Massachusetts Institute of Technology,
Cambridge; NASAís Marshall Space Flight Center, Huntsville,
Ala.; the University of Hawaii, Honolulu; the University of Birmingham,
U.K.; the University of Wollongong, Australia; the Space Research
Organization Netherlands; the University of Rome, Italy; and
the Russian Academy of Sciences. The results will be published
in an upcoming issue of the Astrophysical Journal.

The ACIS instrument was built for NASA by the Massachusetts Institute
of Technology, Cambridge, and Pennsylvania State University,
University Park. NASA's Marshall Space Flight Center in Huntsville,
Ala., manages the Chandra program. TRW, Inc., Redondo Beach,
Calif., is the prime contractor for the spacecraft. The Smithsonian's
Chandra X-ray Center controls science and flight operations from
Cambridge, Mass.Web Links